The present invention relates to the push-push mechanism of a pushbutton operating shaft which may be included in a switch, a volume control or the like used in a radio set or any other audio equipment. More particularly, it relates to the push-push mechanism of a pushbutton operating shaft which can perform a reliable operation with a simple construction.
In recent years, the use of automobiles has increased remarkably, and the use of car radios has similarly increased. A car radio is typically installed in a place comparatively near the body of the driver so that tuning and adjustments of sound volume and the like can be made easily during driving. Unfortunately, however, a part of the body can inadvertently touch one of the controls of the radio, such as the operating shaft for the volume control during driving of the car. Thus, the control may be actuated unintentionally to, for example, turn the operating shaft of the volume control to increase the sound volume unexpectedly or, conversely, establish an inaudible sound level. Such an unexpected happening can, of course, lead to a serious traffic accident, especially if in occurs in traffic. As a safety measure, accordingly, a so-called push-push mechanism for a pushbutton operating shaft has been proposed and put into practical use. In, for example, the aforecited volume control for the sound volume adjustment, the operating shaft is pressed inwardly of the car radio upon completion of adjustment so as not to project considerably from the surface of a panel, and it extends outwardly form the car radio only when operation is necessary.
FIG. 1 is a view for explaining a prior-art push-push mechanism for a pushbutton operating shaft of the type specified above. A frame 102 which receives the push-push mechanism is coupled to the rear of the body 101 which contains the volume control mechanism, and an operating shaft 103 which extends through the body 101 is slidably protruded into the frame 102. The rear end part of the operating shaft 103 is provided with an engaging groove 103a, which holds an engaging portion 104c of a slider 104. The slider 104 may be slid by depressing the operating shaft 103, and it is formed with a heart-shaped cam groove 104a in its upper surface and a recess 104b in its lower surface. A leaf spring 105 is interposed between the top of the frame 102 and the cam groove 104a, and one end of a generally Z-shaped driving pin 106 is urged by the leaf spring 105 into resilient engagement with the cam groove 104a. A resetting coiled spring 107 urges the slider 104 in the direction reverse to the depressing direction of the operating shaft 103 and is retained between the recess 104b and the rear side plate of the frame 102. The other end of the driving pin 106 is pivotally supported in a hole 105a which is provided in the leaf spring 105.
Accordingly, upon depressing the pushbutton operating shaft 103, the slider 104 is moved by this depressing operation, and one end of the driving pin 106 is moved along the heart-shaped cam groove 104a. Upon releasing the depressing force, the depressed state of the operating shaft 103 of the pushbutton is locked by the cooperation between the driving pin 106 and the cam groove 104a, as well known in the art. Upon depressing the operating shaft 103 again, the locked state is released and the operating shaft 103 is urged outwardly by the spring 107. Numeral 108 indicates a cover which overlays the open surface of the frame 102 and which is fixed by protuberant pieces 102a of the frame 102.
In prior art push-push mechanisms as described above, one end of the driving pin 106 must move both vertically and laterally with respect to the moving direction of the operating shaft 103 and therefore some play is required. In addition, the slider 104 needs to move smoothly within the frame 102 in engagement with the engaging groove 103a formed at the rear end part of the operating shaft 103, and therefore some play must be provided between it and the frame 102. For this reason, it is difficult to set the stop position for the operating shaft 103, as determined by the heart-shaped cam groove 104a and the driving pin 106, at a high dimensional accuracy, and inevitably safety must be allowed for in design, which has led to the disadvantage that the switch, the sound volume or the like becomes large in size. Moreover, since the driving pin 106 is easy to move, there has been the disadvantage that upon action of vibrations, an impact or the like, the front end of the driving pin 106 rises, so the operation cannot be carried out smoothly and reliably.